Rhesus monkeys infected with various strains of SIV/Delta have been evaluated for the presence of arthritis. During 1993, 126 cases were studied. In 55 animals, the joints were normal, while 27 had hyperplastic synovium, 40 had mononuclear inflammatory cell infiltration, and 4 had syncytial cell infiltrates. The overall incidence of arthritis in SIV-infected rhesus is thus 57%. To evaluate the possibility of mucosal immunity playing a role in the development of reactive joint disease, a study of the ileum, colon, mesenteric lymph nodes and synovium of serially sacrificed SIV-infected animals was initiated to evaluate the early events of the disease. The role of ileal mucosal immunity will be evaluated concerning IgA and IgG production. Serially sacrificed infected rhesus have been completed at 1, 3, 6, and 14 days PI. Morphometric evaluations of the ileum, colon, and mesenteric nodes was combined with immunohistologic definition of CD4, CD8, macrophage, IgA, and IgG producing cells. Several specific sites are being measured, such as lamina propria of the ileum and colon, and cortical sinusoids of the mesenteric lymph nodes. Germinal center size remained constant in the "slow progressing" animals; the fast progressors had a progressive increase in size from 0.01 to 0.04 mm2 . The CD4+ cells of the cortical sinusoids increased from 60 cells/ mm2 to 401 cells by Day 3 and decreased to 40 cells/mm2 at Day 14. In the lamina propria of the colon and ileum, the CD4+ cells progressively decreased to 50% of the original values by Day 14. The CD8+ cells of the mesenteric lymph node to decreased approximately one-half by Day 14 of the number seen at Day 1. The CD8+ cells of the lamina propria of the ileum and colon remained constant at all sampling periods. IgA + cells of the lamina propria decreased from Day 1-- 183 cells/mm2 to 90 cells/mm2 at 14 days, thus indicating a marked reduction in the number of mucosa producing IgA cells and reduction in non-specific mucosal immunity of both the ileum and colon. Virus positive cells were related to the "rapid progressors" in the mesenteric lymph node, ileum, and colon.